I guess that if an intruder cuts the phone wire prior to entry the cell phone call feature is no longer available. Is this why an internal or external siren should be considered?

That is true, if a burglar cut the phone line then it would be impossible for the panel to call out through the phone line.

This is why we recommend our Simon XT Plug & ProtectTM solution. This ensures that your alarm system will communicate with the central station. With our LiveWatch Complete service, you can have 5 wireless Z-Wave sirens, in addition to being able to log into your system remotely and access up to 4 video cameras.